The present invention relates to a sensor for measurement of change of a magnetic field, for example when indicating the distance to a magnetically conductive object or between magnetically conductive objects.
Determination of the distance to a magnetically conductive object by means of measurement of the change of a magnetic field is already known.
In German Offenlegungsschrift No. 2,102,058, for example, is shown a nozzle which provides a ferromagnetic wire with a casing. Inserted in the nozzle is an annular magnet which generates a magnetic field flowing from the north pole of the magnet via the wire, a pole shoe, a focussing point having a measuring element and back to its south pole. By means of the measuring element a direct measure is thus obtained of the distance between the wire and the pole shoe. The disadvantage of this form of direct measurement is that the measuring element must be located in the magnetic path and that such a location is not always possible to attain on account of, for example, lack of space or because the measuring element may be damaged.
Continuous monitoring of the distance to a drive shaft with a large axial path with the aid of Hall generators is also known, for example from German Offenlegunsschrift No. 1,473,854. According to this publication the shaft is provided with magnetic strip located at a mutual distance from each other and having opposite polarities, and at a fixed distance from the shaft a Hall generator is subsequently located. The Hall generator thus directly measures the magnetic field between the magnetic strips and by this means senses the axial play. In this design it is necessary that the shaft be manufactured of a non-magnetic material.
Shown in Swedish Pat. No. 308,211 is a device in a disc mill to measure the distance between the discs. This device utilizes magnets and coils located in the peripheries of the discs and the magnetic field between them constitutes an indication of the distance between them magnet and the coil and thus between the discs. Consequently the mutual distance between the discs is measured at their peripheries but not the air gap between the grinding segments, and since a pulsing output signal is obtained from the coils, it has been deemed necessary to utilize an expensive oscilloscope for reading the output signal. This prior art device is also an example of directly sensing a magnetic field the size of which depends on the distance. Any wear on the magnet thus influences the measured distance.
Another known method of measuring distance is shown in Swedish Pat. No. 359,644. This method utilizes direct measurement of the magnetic field between the objects concerned and a comparative measurement in a second magnetic circuit. This method, also, has the disadvantage in principle that the measuring probe must be inserted directly in the field between the objects the distance between which is to be determined.
According to U.S. Pat. No. 3,849,724 two signals are utilized to determine the distance between two magnetic objects. Here, too, it is preferred that at least one sensor is located in the main magnetic field but also a second sensor is located in the leakage magnetic field. The disadvantage of this type of measuring method is the same as for all other methods mentioned above and the method also requires complicated electronic equipment.
Shown in Swedish Patent application No. 76 12438-7 is a method for detecting the position of moving parts by means of a Hall generator. This prior art method utilizes two permanent magnets and a Hall generator placed between the magnets. This prior art method as the disadvantage that it cannot be used to measure the distance to an object or the distance between two objects and also requires the presence of two permanent magnets in the symmetry fields of which the Hall generator must be located. Similar symmetry arrangements with two permanent magnets are also utilized in other contexts as signal sensors, see for instance the article "Kontaktloser Signalgeber mit beruhrungsloser Betatigung durch Eisenteile" by H. J. Lippmann on pages 367-372 in ETZ-A 83 (1962):11. The type of signal sensor shown in this last-mentioned publication can only be utilized if an object is approaching the signal sensor and not to measure the distance to an object or between two objects and likewise utilizes two permanent magnets.
The main object of the present invention is to provide a simple and uncomplicated sensor for determination of the distance to a magnetically conductive object or between magnetically conductive objects while avoiding complicated electronics.
Another object of the invention is to provide a sensor which does not need to have the field sensor located in the main magnet field and which can participate in the wear which takes place, for instance between grinding discs, without the measuring result being changed to any appreciable extent.